Wafer-scale arrays of imaging systems offer the benefits of vertical (i.e., along the optical axis) integration capability and parallel assembly. Conventional wafer-scale imaging systems are known, however, to suffer from a lack of precise integration associated with parallel assembly. Offsets of optical elements, due to thickness non-uniformities or systematic misalignment (including translations and rotations) of optical elements relative to an optical axis, may significantly degrade the integrity of one or more imaging systems throughout the array. In rotational misalignments of the common base, the offsets of optical elements are proportional to the distance from the substrate's center of rotation to the optical element. For example, a rotational misalignment of one milliradian (0.0573 degrees), for a 300 millimeter substrate, can result in a 150 micron offset for optical elements near the substrate's perimeter, thereby significantly reducing imaging system quality. Conventional wafer-scale technologies typically enable alignment only at mechanical tolerances of several microns, and do not enable alignment at optical tolerances (i.e., on the order of a wavelength of electromagnetic energy of interest), which would be the alignment accuracy required for precise imaging system manufacture.
One high-precision wafer-scale array system is disclosed in Patent Cooperation Treaty Publication No. WO/2008/020899, filed 17 Apr. 2007, which is incorporated by reference herein. WO/2008/020899 discloses several embodiments related to the alignment of one or more fabrication masters with a vacuum chuck and/or a common base to simultaneously form a plurality of high quality imaging systems. A fabrication master may include features that form optical elements, and may be used to form an array of optical elements on the common base. As disclosed in WO/2008/020899, the common base can be an array of image detectors and/or the common base can support an array of layered optical elements thereon. The fabrication master and the vacuum chuck may thus be used to form an array of imaging systems supported on the common base.
Precise alignment of an imaging system fixturing component, such as the fabrication master, chuck, or common base, or alignment of two or more of such imaging system fixturing components together, is explained with reference to FIG. 1. FIG. 1 shows an oblique perspective view of a cylindrical object 100, whose alignment is defined according to a coordinate system 102. Cylindrical object 100 represents one of the aforementioned imaging system fixturing components. When affixed to a point in space, or to a second object (not shown), coordinate system 102 defines the translations and rotations of object 100 with respect to the point in space or second object. Translations of object 100 are defined by the three orthogonal x-, y-, and z-axes as indicated by directional arrows X, Y, and Z, respectively; and rotations of object 100 are defined by rotations about each of the three orthogonal axes as indicated by directional arrows A, B, and C, respectively (also referred to as pitch, roll, and yaw). Alignment of object 100 in all six directions (i.e., A, B, C, X, Y, and Z) is referred to as full kinematic alignment, while alignment of the object in less than all six directions is referred to as pseudo-kinematic alignment.
In order to facilitate precise and accurate alignment between object 100 and the second object, each can include visual and/or structural alignment features. Visual alignment features (referred to herein as “alignment marks” or “fiducial marks”) are generally markings on each of the objects that facilitate the visual alignment of the objects with respect to each other. Structural alignment features (referred to herein as “alignment structures”) on each object can mechanically assist in the alignment of the objects with respect to each other, or allow the objects themselves to self-align when mated. Corresponding alignment structures/marks on two objects are collectively referred to as a set of alignment structures/marks.
More specifically, a set of alignment structures/marks that indicates displacement between two or more objects with respect to all six degrees of freedom with respect to coordinate system 102 (i.e., displacement along the X, Y, and Z axes and rotation about each of the X, Y, and Z axes) will enable full kinematic alignment. On the other hand, a set of alignment structures/marks that indicates displacement between two or more objects with respect to only some, but not all six, degrees of freedom in coordinate system 102 (e.g., displacement along the X, Y and Z axes but not rotation about the axes) may only enable pseudo-kinematic alignment.
When used in forming optical elements for an imaging system on the common base, the fabrication master should be accurately and precisely aligned with the common base and/or the vacuum chuck that supports the common base. For example, if the common base includes an array of image detectors supported thereon, then optical elements formed using the fabrication master should be aligned accurately and precisely with their corresponding image detectors. Alignment structures on each of the fabrication master, the vacuum chuck, and the common base, in this example, may facilitate the alignment of the fabrication master with respect to the vacuum chuck and/or common base during the formation of the optical elements.
One prior art example of a set of alignment structures is illustrated in FIGS. 2 and 3. FIG. 2 is a top perspective view of a vacuum chuck 200 supporting a common base 202 thereon, and FIG. 3 is a top perspective view of a fabrication master 300 configured to be engaged with vacuum chuck 200 and common base 202. Common base 202 includes a generally planar surface 204. In FIGS. 2 and 3, as well as in the other figures throughout this disclosure, specific instances of an individual element of a plurality are referred to by use of a numeral in parentheses (e.g., convex alignment element 306(1)) while numerals without parentheses refer to the element generally (e.g., convex alignment elements 306). Common base 202 can be configured to support an array of image detectors, such as complementary-metal-oxide-semiconductor (CMOS) image detectors.
Continuing to refer to FIGS. 2 and 3, fabrication master 300 includes a plurality of features 302 configured for forming optical elements therefrom; only three of such features 302 are labeled in FIG. 3 for illustrative clarity. Fabrication master 300 can be used to form on surface 204 a plurality of optical elements corresponding to features 302. For example, by disposing a surface 304 of fabrication master 300 proximate to surface 204 of common base, with a moldable or shapeable material disposed therebetween, features 302 can be used to mold optical elements on surface 204.
As shown in FIG. 2, vacuum chuck 200 includes cylindrical alignment elements 206(1), 206(2) and 206(3) as alignment structures. Fabrication master 300 includes convex alignment elements 306(1), 306(2) and 306(3) as respectively corresponding alignment structures. When engaged in a cooperative manner, cylindrical elements 206 and convex elements 306 define the translation and tilts (i.e., rotations about the two axes that are in the plane of surface 204 or surface 304) between vacuum chuck 200 and fabrication master 300. However, rotational movement about the axis normal to surface 204 or surface 304 (e.g., as illustrated by arrow 210 in FIG. 2) between the two imaging system fixturing components can still be effected by active, external forces, such as those from an operator or a machine (not shown). Accordingly, the set of alignment structures defined by cylindrical alignment elements 206 and convex alignment elements 306 enables only pseudo-kinematic alignment in the example shown in FIGS. 2 and 3.
However, angular alignment of fabrication master 300 with respect to vacuum chuck 200 can be provided by including fiducial marks on both objects. Such fiducial marks act to indicate visually when fabrication master 300 is in proper angular alignment with respect to vacuum chuck 200. Although fiducial marks can facilitate such angular alignment, they do not generally contribute to self-alignment (also referred to as passive alignment) between the objects. An operator and/or machine typically rotates one or both of vacuum chuck 200 and fabrication master 300 until the fiducial marks visually indicate proper angular alignment. In this example, the fiducial marks are represented by radial lines 310(1) and 310(2) configured for alignment with corresponding radial lines (not shown) on vacuum chuck 200. Concentric ring 312 may also serve as a fiducial mark by aligning with perimeter 212 of common base 202. Verniers 208(1) and 208(2) on vacuum chuck 200, and corresponding verniers 308(1) and 308(2) on fabrication master 300, illustrate another example of such cooperative fiducial marks for facilitating angular alignment.
FIG. 4 shows a cross-sectional view of a prior art replication system 400 including alignment structures 410 and 412 on a vacuum chuck 402 and alignment structure 414 on fabrication master 404. Alignment structures 410 and 412 are shown engaged with a corresponding alignment structure 414. A common base 406 is secured onto vacuum chuck 402 and, by use of alignment structures 410, 412, and 414, aligned with respect to fabrication master 404. As shown in FIG. 4, fabrication master 404 includes a plurality of structures 408 for defining optical elements. A press 416 is used in replication system 400 to retain fabrication master 404 against common base 402.
Still referring to FIG. 4, alignment structures 410, 412, and 414 cooperate to constrain the degrees of freedom in translations and tilts of fabrication master 404 with respect to vacuum chuck 402. In this example, rotation of fabrication master 404 with respect to vacuum chuck 402 is still primarily defined by an active external alignment force (e.g., press 416). Accordingly, the set of alignment structures of replication system 400 may enable only pseudo-kinematic alignment of fabrication master 404 with respect to vacuum chuck 402.
FIG. 5 shows a perspective view of another example of a prior art alignment system 500, which includes a vacuum chuck 502, a fabrication master 504 and a vision system 506. In one process of controlled engagement, fabrication master 504 and vacuum chuck 502 are aligned relative to one another in the θ-direction before being brought into engagement in the Z-direction. In the example shown in FIG. 5, alignment in the θ-direction is sensed electronically by vision system 506 to determine the relative positional alignments between an indexing mark 508 on fabrication master 504 and an indexing mark 510 on vacuum chuck 502. Vision system 506 produces a signal that is sent to a computer processing system (not shown), which interprets the signal to provide positional control of one or both of vacuum chuck 502 and fabrication master 504 relative to each other. Displacement or translation along the Z- and R-directions is controlled by mechanical alignment structures, such as by the alignment structures shown in FIGS. 2 through 4. Accordingly, the combination of aligning structures and features illustrated in FIG. 5 enable pseudo-kinematic alignment of vacuum chuck 502 with respect to fabrication master 504 in the θ, R, and Z directions.
FIG. 6 shows a cross-sectional view of another exemplary set of prior art alignment structures. In this example, a fabrication master 602 is engaged and mated with a vacuum chuck 600, with a common base 604 being secured onto vacuum chuck 600. The alignment structure on vacuum chuck 600 is a v-groove 606 formed on or into a surface 608 of vacuum chuck 600. As shown in FIG. 6, v-groove 606 is formed on a surface 608. More specifically, a convex protrusion 610 cooperatively aligns with v-groove 606 when fabrication master 602 and vacuum chuck 600 are engaged. While v-groove 606 and convex protrusion 610 limit translation and tilt between fabrication master 602 and vacuum chuck 600 when engaged, this set of alignment structures generally defines only pseudo-kinematic alignment between fabrication master 602 and vacuum chuck 600, and not full kinematic alignment in and of themselves.